Method of treating pulverulent material

Abstract: A method is described for the manufacture of coarse and fine products, such as normal and ultra-fine cement, in a plant comprising at least a mill (1) classifying separator (3), a separation cyclone (5) and a dust collector (7). By this method some of the dust-laden conveying gas which is extracted via the cyclone gas outlet duct (11) is diverted to and deducted in the dust collector (7), and the dust from the dust collector (7) is utilized as the fine product, whereas the cyclone material from the material outlet (9) of the cyclone is utilized as coarse product. Hence it is possible to manufacture ultra-fine cement in existing grinding plants in a mechanically simple and operationally inexpensive way, while normal cement is simultaneously manufactured, without having any significant impact on the mill operation.

Description: The present invention relates to a method (hereinafter referred to as of the kind described) for treating pulverulent material to produce coarse and fine products, such as normal and ultra-fine cement on the basis of cement materials such as cement clinker, gypsum, slag, limestone, pozzuolana, flyash etc., in a plant comprising at least a mill, a classifying separator, one or several separation cyclones and a dust collector, by which method the material is fed to and ground in the mill, and subsequently conveyed to the separator in which it is divided into a coarse fraction which is returned to the mill and into a fine fraction which suspended in a conveying gas is conducted to the cyclone in which the material/gas suspension is separated into cyclone material which is discharged via the material outlet of the cyclone and into dust-laden conveying gas which is discharged via the gas outlet duct of the cyclone, and whereof a partial quantity is subsequently conducted via a further duct to, and dedusted in, the dust collector. Apparently, there is a strong trend towards incorporation of very fine types of cement, as for example the so-called ultra-fine cement, in the product range of the cement manufacturers. By ultra-fine cement is meant cement having fineness in the range of 600-1000 m 2 /kg, whereas the fineness of normal cement is in the range of 300-500 m 2 /kg. However, in actual practice it is difficult to manufacture cement having the degree of fineness which characterizes ultra-fine cement. When attempting to manufacture ultra-fine cement by means of open-circuit grinding, the tendency towards dry clogging will often be so pronounced as to result in a substantial deterioration of the grinding economy, and, further, it will often be impossible to manufacture cement having a fineness in excess of 800 m 2 /kg. It is, however, possible to manufacture cement having such a fineness in closed-circuit plants, but this has the disadvantage that the circulating load or the circulation factor across the separator will be quite substantial, which in turns means that the specific power consumption of both the separator and the fan will be very high. Plants of the kind mentioned above are known for example from DE 3908752-Al and DE 3921823-Al. In both specifications methods are described by which cyclone material and dust from the dust collector are intermixed. EP-A-0519535 discloses a method for separating such materials, whereby two products of differing fineness are produced and can be used either separately or combined. According to the invention a method of the kind described is characterized in that the dust is extracted from the dust collector and utilized as a fine product; in that the cyclone material is utilized as a coarse product; in that the fineness and/or the chemical composition of the fine product is adjusted by addition of dust-loaded conveying gas extracted from and/or before the cyclone, and optionally also by addition of cyclone material. The invention is applicable to the treatment of other materials, such as coal. However of great interest is when the material is a cement material, in which case the fine product can be used as a basis for an ultra-fine cement while the coarse product is a normal cement. The expression "as the basis for" is intended to cover an ultra-fine cement where the dust from the dust collector constitutes between 50 and 100%. In this manner ultra-fine cement can be manufactured in existing grinding plants according to a mechanically simple and operationally inexpensive method, while normal cement is simultaneously manufactured, and without having any significant impact on the mill operation. The present invention is based on the recognition that the cyclones, which are normally utilized in the cement-making industry, have a separation efficiency ranging between 90 and 98%, which means that between 2% and 10% of the material being fed to the cyclone, substantially as dust, is not separated but discharged via the gas outlet duct together with the conveying gas. Since, at the same time, the so called cut size of these cyclones is very low, the fineness of this material or dust will be exceedingly high, being in the range of 800-1500 m 2 /kg. The dust-loaded conveying gas when extracted from the cyclone and used to adjust the fineness and/or chemical composition of the fine product may be extracted by a fan via one or several openings - being arranged eccentrically relative to the cyclone gas outlet duct - at the top of the cyclone, while the dust-loaded conveying gas when extracted before the cyclone and used for adjusting the fineness and/or the chemical composition of the fine product may be extracted via an opening in the cyclone inlet duct by means of the same fan. In case of critical fan wear, the dust in the extracted, dust-loaded conveying gas may be separated in a secondary dust collector, subsequently being conveyed past the fan and recharged into the conveying gas after the fan. To achieve homogeneity of the fine product, the dustloaded conveying gas, and/or, the cyclone material, may be charged to the further duct and there intermixed with the dust-laden conveying gas from the cyclone gas outlet duct prior to the introduction into the dust collector which may be effected in the duct leading to the dust collector. The fineness and/or the chemical composition of the fine product may also be adjusted by some of the cyclone material being charged to and intermixed with the dust subsequent to the extraction of the dust from the dust collector. The invention will in the following be described in further details with reference to the accompanying diagrammatical drawings, in which:- Fig. 1 shows a plant for carrying out the method according to the invention; and, Fig. 2 shows a plan view of details of the plant in Fig. l. The plant shown in Fig. 1 comprises a mill 1, a separator 3, a cyclone 5 and a dust collector 7 in the form of a precipitator. Fresh material for the cement manufacture is fed to and ground in the mill 1, which, in the example shown, is a roller press. The ground material is subsequently conducted for example by means of an elevator 2 to the separator 3 in which it is divided into a coarse fraction which is returned to the mill 1 and into a fine fraction which is suspended in a conveying gas and conducted to the cyclone 5 via a duct 4. In the cyclone 5 the majority of the fine fraction from the conveying gas is separated in the form of cyclone material which is discharged via the material outlet 9 of the cyclone 5 and passed on via a duct 20. The conveying gas containing the residual amount of very fine material or dust is discharged via the gas outlet duct 11 of the cyclone 5 and is recirculated via a fan 8 and a duct 23 to the separator 3. Some of this dust-laden recirculating gas, corresponding to the amount of false air and any fresh air, drawn or fed into the plant, is conducted away via a duct 12 and dedusted in the dust collector 7. Fresh air in the form of atmospheric air or preheated air may be supplied to the system via a duct 10. According to the invention the dust is extracted via a duct 24 from the dust collector 7, which, in the example shown, consists of a precipitator, and the dust is utilized as the basis for ultra-fine cement, whereas the cyclone material from the duct 20 is utilized as normal cement. In the plant shown, the fineness and/or chemical composition of the ultra-fine cement can be adjusted in several ways. Dust-loaded conveying gas is extracted before and/or from the cyclone 5 via openings 15, 13 provided in the duct 4 and in the top of the cyclone 5, respectively, as shown in Fig. 2, and via a fan 17 and a possible dust collector 19 added to the diverted, dust-laden conveying gas in the duct 12. Cyclone material may also be added via duct 21 directly to the dust in duct 24, or it may be added via duct 22 to the diverted, dust-laden conveying gas in the duct 12. Fig. 2 shows two openings 13 at the top of cyclone 5, but the cyclone 5 may incorporate a plurality of corresponding openings. The dust-loaded conveying gas is extracted from one or several of the openings 13, 15 dependent upon the fineness of the dust being required to obtain a desired fineness of the finished ultra-fine cement. The dust which is extracted via the opening 15 will thus have the coarsest characteristics and will typically lie within the range 300-500 m 2 /kg, whereas the dust being extracted via the opening 13a will lie within the range 350-600 m 2 /kg and the dust being extracted via the opening 13b will lie within the range 400-800 m 2 /kg. In an alternative embodiment of the method according to the invention the dust-loaded conveying gas which is extracted via the openings 13 and 15 may be charged directly into the gas outlet duct 11 of the cyclone 5 via duct 25. This solution would require installation of control dampers in the various ducts. But, normally, this solution is not very attractive, since it will lead to an extremely high dust concentration in the gas which is drawn through the fan 8 and whereof a partial quantity is recirculated to the separator 3. The plant described above is not resstricted to the manufacturing of cements with different degree of fineness. In fact, the plant may be used for the treatment of any kind of pulverulent material, where a particularly high fineness is required. Thus, the plant may with advantage be used for fine grinding coal, e.g. to be fired into a cement kiln.

Claims: A method for treating pulverulent material in a plant comprising at least a mill (1), a classifying separator (3), one or several separation cyclones (5) and a dust collector (7), by which method the material is fed to and ground in the mill (l), and subsequently conveyed to the separator (3) in which it is divided into a coarse fraction which is returned to the mill (1) and a fine fraction which suspended in a conveying gas is conducted to the cyclone (5) in which the material/gas suspension is separated into cyclone material which is discharged via the material outlet (9) of the cyclone and into dust-laden conveying gas which is discharged via the gas outlet duct (11) of the cyclone, and whereof a partial quantity is subsequently conducted via a further duct (12) to, and dedusted in, the dust collector (7), characterized in that the dust is extracted from the dust collector (7) and utilized as a fine product; in that the cyclone material is utilized as a coarse product; and in that the fineness and/or the chemical composition of the fine product is adjusted by addition of dust-loaded conveying gas extracted from and/or before the cyclone (5). A method according to claim 1, in which the fineness and/or the chemical composition of the fine product is also adjusted by addition of cyclone material. A method according to claim 1 or claim 2, in which the dust-loaded conveying gas extracted from the cyclone (5) and used to adjust the fineness and/or chemical composition of the fine product is extracted by means of a fan (17) via one or several openings (13) - eccentrically arranged relative to the gas outlet duct (11) - in the top of the cyclone (5). A method according to claim 2 and any one of the preceding claims, in which the dust-loaded conveying gas extracted before the cyclone (5) and used to adjust the fineness and/or chemical composition of the fine product is extracted via an opening (15) in the inlet duct of the cyclone (5) by means of a fan (17). A method according to claim 3 or claim 4, in which the dust in the extracted dust-loaded conveying gas is separated in a secondary dust collector (19), and conveyed past the fan (17) and re-charged into the conveying gas after the fan (17). A method according to any one of claims 2 to 5, in which the dust-loaded conveying gas and/or the cyclone material is charged to the further duct (12) and there intermixed with the dust-laden conveying gas from the gas outlet duct (11) of the cyclone (5) prior to the introduction into the dust collector (7). A method according to claim 2, in which some of the cyclone material is charged into and intermixed with the dust extracted from the dust collector (7) for producing fine product having a desired fineness and/or chemical composition. A method according to any one of the preceding claims, in which the material is coal. A method according to any one of claims 1 to 7, in which the material is a cement material, the fine product is used as the basis for an ultra-fine cement, and the coarse product is a normal cement.

Inventors: FOLSBERG JAN, FOLSBERG, JAN
Applicants: SMIDTH & CO AS F L, F.L. SMIDTH & CO. A/S
Application year: 1994
Main classification: B02C23
IPC codes: B02C21, C04B7, B02C23, B04C9